The present invention relates to a method of preparing membranes having ion-exhange properties for use in electrolyisis and electrolytic cells.
Many types of ionic or non-ionic membranes have been proposed for many years as separator diaphragms in electrolysis, electrodialysis, and other applications using electric cells with electrodes.
Among the various applications contemplated, one of them is of great importance, namely, the electrolysis of aqueous saline solutions, particularly solutions of sodium chloride (NaCl) for the production of chlorine and caustic soda. The importance of these processes comes from the fact that industry requires a large amount of chlorine and caustic soda so that the NaCl electrolysis units are necessarily large. Furthermore, in view of the quantities manufactured, the slightest gains in chemical, electrical, or energy yield are highly desirable. However, the production and improvement of NaCl electrolysis membranes present very difficult problems, since the simultaneous presence of chlorine and high concentrations of caustic soda in the hot electrolyte constitute corrosion conditions which few membranes are capable of withstanding.
In addition to the corrosion-resistance properties just mentioned, it is advantageous to use cation-selective membranes, i.e., cation exchangers, since they make it possible to obtain caustic soda which is less contaminated with chloride and, potentially, better electrical yields.
When the membranes used are ion-selective, it is, in general, more difficult to obtain the ability that they withstand corrosion, than when non-ionic membranes are employed.
It is accordingly an object of the present invention to provide a method of preparing electrolytic cell membranes having ion exhange properties. One such method comprises impregnating a porous sheet comprising a polyfluoroolefin, asbestos fibers and an inorganic pore-forming agent with a solution comprising (i) a compound selected from the group consisting of acrylic acid and methacrylic acid and (ii) at least one other non-ionic monomer having at least one &gt;C.dbd.CH.sub.2 group, withdrawing said sheet from contact with said solution, polymerizing the monomers which become impregnated in said sheet so that the pores of the sheet contain the resulting acrylic polymer, eliminating said inorganic pore-forming agent to form pores in said sheet, and swelling the acrylic polymer so that the pores of the sheet are closed by the polymer.
Other objects of the invention will be apparent to those skilled in the art from the present description.